1. Field of the Invention
The invention present relates to a hydraulic bearing that supports a rotating shaft or the like.
2. Description of the Related Art
FIGS. 12(A) to 12(C) are three partially developments showing inner surfaces of bearing metals which constitute radial hydraulic bearings according to the prior arts. Plural hydrostatic pockets 21 having quadrilateral grooves as shown by FIGS. 12(A) and 12(C) or U-shaped grooves as shown by FIG. 12(B) are formed on each inner surface of the bearing metals along a rotational direction of a rotating shaft. An oil-supplying hole 23 is formed in each hydrostatic pocket 21. Land portions 7 formed on the inner surface of the bearing metal except for the hydrostatic pockets 21 are for generating hydrodynamic pressure.
Here, the hydraulic bearing is distinguished to two types in which one is a separated type as shown by FIG. 12(C), and the other is a non-separated type as shown by FIG. 12(A) or 12(B) in accordance with a shape of the land portion 7. The land portion 7 of the non-separated type is circumferentially formed on all of the surface of the bearing metal. On the other hand, the land portions 7 of the separated type are separated along with rotational axis of the rotational shaft by drain grooves 22 that are formed between adjacent two hydrostatic pockets 21. In the aforementioned hydraulic bearings, when pressure-adjusted lubricant oil is supplied to the hydrostatic pockets 21 through the oil-supplying hole 3, the rotating shaft is supported hydrostatically by the filled lubricant oil between the hydrostatic pockets 21 of the bearing metal and an outer surface of the rotating shaft. Simultaneously, the lubricant oil is filled between the land portion 7 and the rotating shaft. With the filled lubricant oil, when the rotating shaft is rotated in the bearing metal, the rotating shaft is supported hydrodynamically by wedge effect that is generated between the land portions 7 and the outer surface of the rotating shaft.
Then, at the non-separated type bearing, especially in a case of U-shaped hydrostatic pockets 21 such as shown by FIG. 12(B), since an area of each land portion 7 is large and continuously, a large amount of hydrodynamic pressure is generated. Therefore, the non-separated type bearing is effective in high rigidity and high damping effect. However, in case of high rotating speed, a great heat due to fluid friction is generated at the land portions 7. The great heat causes thermal expansion of the bearing metal, and a clearance between the bearing metal and the rotating shaft decreases. As the result, calorific value by fluid friction increases, and thermal expansion of the bearing metal increases. This causes such a vicious circle that deteriorate the performance of the bearing.
On the other hand, at the separated type bearing, heat generated at the land portions 7 is restrained because it is easy to drain the lubricant oil by existence of the drain grooves 22. However, existence of the drain grooves 22 causes deterioration of the rigidity because each land portion 7 is separated and small. Moreover, the separated type bearing tends to cause cavitation.
We, Toyoda Koki Kabushiki Kaisha, applied Japanese Patent Application No. 2000-289889 filed on Sep. 25, 2000 which resolves two problems above. According to that application, plural drain holes are formed at land portions of bearing metal. One end of each drain hole is opened on the land portion, and the other end of each drain hole is connected to a tank. Therefore, since the area of the land portions is essentially as large as the non-separated type bearing as shown by FIGS. 12(A) and 12(B), the hydraulic bearing is effective in high rigidity similar to the non-separated type bearing. In addition, since the lubricant oil at the land portions is drained to the tank through each drain hole, the hydraulic bearing has low temperature rise that is close to the separated type bearing as shown by FIG. 12(C).
The hydraulic bearing above, however, the land portions are connected to the tank released to the atmospheric pressure. In connection with the eccentricity of the rotational shaft relative to the bearing metal, negative pressure may be generated at one region in the hydraulic bearing. Then, air in the tank is suctioned into the land portions through the drain holes by negative pressure. More rotational speed of the rotational shaft, negative pressure generates easier even though the eccentricity is less. When rotational speed of the rotating shaft become large, it is facilitates to generate negative pressure even if the eccentricity of the rotating shaft is small.